1. Field of the Invention
The invention relates to a method for processing and transmitting digital data in a mobile telephone network.
It applies more particularly to a mobile telephone network that complies with the “GSM” standard (the acronym for Global System for Mobile Communication, operating in the 900 MHz bandwidth).
The invention also relates to an embedded system equipped with a microchip for implementing the method.
Within the scope of the invention, the term “network” should be understood in its most general sense. It includes the transmission components of the network themselves (radio transmission subsystems, transmission cables, microwave radio systems, terrestrial “wired” subsystems, etc.), but also all the systems connected to the mobile telephone network (base stations, station controllers, switching systems, directories, etc., and more generally, all data processing systems and servers connected to the network), including the mobile telephones, equipment or stations in the possession the users (subscribers) of the mobile telephone network.
The latter devices can be portable telephones or more complex terminals, for example a terminal that combines the telephone and organizer functionalities. For simplicity's sake, without in any way limiting the scope of the invention, these devices will hereinafter be called “mobile telephones.” The mobile telephones are specifically equipped with an embedded system equipped with data processing and storage means, including a functional module known by the abbreviation “SIM” (for “Subscriber Identity Module”). Also for simplicity's sake, it will be assumed hereinafter that the “SIM” module is installed in a smart card. Software for operating the smart card is also provided (called “OS,” for “Operating System”).
2. Description of the Related Art
In the current state of the art, the mobile telephones of GSM networks are no longer used just for telephoning. They can also be used to process and send digital data, particularly in the form of short messages (a service called “GSM-Data”). These messages typically have a length of 160 septets or 140 octets, depending on the applications.
Recently, we have seen the appearance of a standardized technology called “SIM Toolkit.” This technology is already included in the complementary services offered by certain mobile telephone operators of GSM networks. In a practical way, a specific piece of software is implemented in the “SIM” smart card of the telephone.
This standard allows the applications running in the smart card (“SIM”) of the mobile telephones to send commands to the associated mobile equipment for:                displaying a text on the screen of the mobile telephone;        prompting the user to type the text as input in the application;        requesting a telephone call to a number;        requesting the sending of a short message that contains text or data to a server or to another telephone;        requesting the execution of a command in an auxiliary smart card in the mobile equipment;        etc.        
These commands are called “proactive” commands.
The “SIM Toolkit” standard also allows the applications of the “SIM” smart card to react upon reception of a certain number of events issuing from the mobile equipment. These events include:                the selection of the application from a so-called “MENU” command located on a keyboard with which the mobile equipment is equipped;        the reception of a short message sent to an application or to the operating system of the “SIM” smart card;        the expiration of the time programmed into a timer of the mobile equipment;        a request to call a telephone number;        etc.        
The functionalities provided by this standard make it possible to develop a very large number of distinct applications in the smart card, in order to provide users with so-called “value-added” services.
For a more detailed description of “SIM Toolkit” technology, it would be useful to refer to the GSM 11.14 standard.
In summary, in the current state of the art, the “SIM Toolkit” standard consists of developing applications in the smart card that are capable both of controlling the mobile equipment supporting the smart card and of reacting to events issuing from his equipment. Today, this standard has been adopted by the majority of manufacturers of mobile equipment and smart cards. It is a standard that makes it possible to write applications that are both powerful and secure.
“SIM Toolkit” application generally communicates with one or more applications installed in servers, via the short message channel. This channel is independent from the voice channel and is standardized by the GSM standard. The applications, whether they are in stalled in the SIM card or in the server, can both send and receive short messages that contain text or data in binary format. (For more information on short messages in point-to-point mode, see the GSM 03.40 standard.)
However, while the number and nature of the aforementioned value-added GSM services are a priori unlimited, telephone operators and developers of software applications are constantly held back by the limited resources available in the smart card.
In essence, the two main drawbacks of the “SIM Toolkit” technology are;                1) insufficient memory for hosting “SIM Toolkit” applications in the smart cards, since the latter include only several tens of kilobytes in memory; in spite of all the optimizations performed by highly experienced developers of software applications, telephone service operators run up against this lack of memory for loading and/or running large numbers of “SIM Toolkit” applications; and        2) the long response time of smart cards; in essence, this technology requires the smart card, which is equipped with minimal computing power compared to the processors of mobile equipment, especially servers, to perform the most complicated calculations during the execution of the aforementioned applications.        
It is clear that the needs that have arisen run in the opposite direction. First of all, telephone operators want to offer their subscribers the largest possible number of available additional services. Moreover, the operations performed by the smart card must be done in a minimal amount of time. The increase in available applications, to the extent that this increase is possible, also runs the risk of degrading performance by making the processing time even longer, due to mutual interactions.
Despite the substantial advances in smart card technology already seen lately and foreseeable in the near future, the aforementioned limiting factors remain and will remain an important obstacle.
A second standardized technology called “WAP” (for “Wireless Application Protocol”) has recently been proposed. The purpose of this standard is to allow users of mobile telephones to access the Internet from their mobile telephones, via a wireless link.
This technology is not without its drawbacks.
First of all, it is necessary to install a “WAP” browser of a specific type, since it has characteristics different from conventional Web browsers. This browser is installed in the mobile equipment, which definitely has more memory than the smart card, but generally less than a microcomputer and naturally less than a server. Furthermore, although this technology offers the advantage of allowing access to the Internet, it does not cover all the functionalities of SIM Toolkit applications. To give a nonlimiting example, a WAP application cannot, as in the case of SIM Toolkit applications, order telephone calls. Moreover, a WAP application cannot guarantee the same degree of security as that offered by SIM Toolkit applications. In essence, the latter use secret keys stored in the SIM smart cards. For example, a WAP application cannot request the authentication of an application in any server. Moreover, this is not the only security function devolved to the SIM smart card. It also ensures the security and confidentiality of the transmitted information.
In order to provide all of these functions, the smart card stores a certain number of so-called “sensitive” data, encryption algorithms and associated keys, including:                the international subscriber number of “IMSI” (for “International Mobile Subscriber Identity”);        the temporary identity assigned to a mobile station when it moves or “TMSI” (for “Temporary Mobile Subscriber Identity”);        an individual authentication key, an encryption key used to encrypt and decrypt so-called signaling and traffic data on the radio channel and three distinct encryption and decryption or key generating algorithms; and        a number of encryption key sequences or “CKSN” (for “Ciphering Key Sequential Number”) indicating the above valid encryption value, in order to avoid the use of different keys by the mobile station and the network.        
The fact that this sensitive information or data is stored in the smart card and that the corresponding operations are performed in the latter makes it possible to obtain a high degree of security and confidentiality.
Finally, in the current state of the art, the cost of the mobile equipment that supports WAP technology is still high compared to the cost of the equipment that supports SIM Toolkit technology.